This is the renewal of a grant entitled Aging, Macrophage Mediators, and Burn Injury (R01 AG18859) which focuses on the mechanisms by which advanced age alters macrophage phenotype and function. The overall goal of the renewal application is to gain new knowledge about how the inflammatory milieu of aged mice is generated and how it predisposes the aged to aberrant pulmonary and systemic responses after traumatic injury. We know from clinical and experimental evidence that even in the absence of injury, healthy aged subjects have an elevated basal inflammatory state, referred to as inflamm-aging. Moreover, we believe that inflamm-aging is caused by the release of bacterial products from intestinal lumen and that the continuous exposure to these products, as well as low level of inflammatory cytokines, tumor necrosis factor ? (TNF?), interleukin-1? (IL-1?) and interleukin-6 (IL-6), causes a shift in alveolar macrophages to the M2 anti-inflammatory phenotype. A high level of M2 cells in the lungs would render the host unable to combat a pulmonary infectious challenge. We recently reported that after a moderate size scale burn injury, aged mice have greater accumulation of neutrophils in the lung and more edema than their younger counterparts. New and exciting preliminary data reveal that after burn injury alveolar macrophages collected from young mice exhibit an M2 shift, presumably to help resolve inflammation. In contrast, after burn injury, alveolar macrophages from aged mice have a reduced M2 phenotype, which is markedly lower than that of cells obtained from sham injured aged mice. This reduction in M2 cells would allow lung inflammation to persist. From these pieces of evidence, we hypothesize that in the absence of injury alveolar macrophages from aged mice exhibit an M2 phenotype because of the continuous barrage of bacterial products which are elevated as a result of a breach in the integrity of the intestinal epithelial barrier. Moreover, the elevated intestinal damage seen in aged burn-injured mice exacerbates and prolongs post-burn pulmonary inflammation which, in turn, reduces lung function. To test this hypothesis, in Aim 1, we will first determine if advanced age alters the M1/M2 polarization in the lungs of mice in the absence of injury. We will then go on to examine the impact of burn injury on macrophage phenotype. Next, in Aim 2, we will use adoptive transfer and macrophage specific knockout mice to investigate mechanisms by which advanced age alters alveolar macrophage function. In Aim 3, we will examine whether alterations in intestinal permeability after burn injury are more extensive in aged mice and account for the prolonged inflammatory response in the lungs. Finally, we will determine if restoring intestinal barrier integrity and wil reduce pulmonary inflammation and improve lung function. Taken together, these studies will expand on the limited knowledge of how advanced age alters alveolar macrophage function and whether therapeutic interventions targeting the lung and gut can be used therapeutically for the treatment of patients with burn and other critical illnesses.